First, the pictures have to be stored at an external server. If you can't store the picture on a regular web server, you can store it on a image storage service. These have been mentioned here before, though I haven't tried any of them:
When the image is accessible on a server (i.e. you can view the picture with a link) you are ready to post it here. The link have to be to the image itself, not to a page containing the picture. You can find that address in explorer by right clicking on the image, choose "Properties" and then copy the address (could be on two lines) to the image.
Here is an example with my avatar:
The result is this:
It is a very good idea to use the preview button before you post the message, as it is easy to do mistakes. When the preview is ok, you are ready to push the "submit" button.
If you want to make a clickable image, you can do it this way:
which will give this result, pointing to my homepage.